Among the halogenated flame retardants, the halogenated bis-imides are particularly useful because the bis-imide structure confers good thermal stability to the flame retardant, and its incorporation does not detract from the thermal stability of materials such as plastics in which it is used. Moreover, the bis-imides are resistant to photo-degradation. A negative aspect associated with these materials is that they tend to exhibit varying degrees of yellowness. This factor makes them unacceptable for use as flame retardants in certain, especially white, polymeric materials. In addition, the color of a given bis-imide tends to vary from batch to batch. This makes it difficult for a plastics manufacturer to maintain consistency in the colors of his products. The yellow color is believed to be due to the introduction of small amounts of unidentified colored impurities, i.e., "color bodies," during the manufacturing process. In the context of the instant invention, "yellowness index" is determined according to the procedure set forth in ASTM D-1925, entitled "Yellowness Index in Plastics."
U.S. Pat. No. 3,873,567 discloses that N,N'-alkylene-bis-(tetrahalophthalimides) can be used as flame retardants. These compounds can be prepared by imidization of the corresponding halogenated phthalic anhydride with an alkylenediamine in solution using an organic solvent system, but for reasons of cost and safety, an aqueous medium has been preferred. U.S. Pat. No. 4,087,441 discloses the preparation of N,N'-alkylene-bis-(tetrahalophthalimide) compounds in an aqueous suspension, and U.S. Pat. No. 4,092,345 discloses the use of water, along with an alkyl organic acid. In this latter case, the reaction proceeds via an alkylene diammonium-bis-(tetrahalophthalate) salt, which can be isolated. Heating the salt produces the desired ring-closed bis-imide.
Many of the variants of the imidization process carried out in aqueous acid give a solid N,N'-alkylene-bis-(tetrahalophthalimide) which is not the desired white color exhibited by the pure compound, but rather is yellow to one degree or another. Clearly, there is a need for a method to eliminate, or at least decrease, the yellow color associated with N,N'-alkylene-bis-(tetrahalophthalimide) flame retardants.